My 17th Century Dutch isn't so good but I can still look at the illustrations in Johan Picardt's 1660 book "Korte beschyvinge van eenige vergetene en verborgene Antiquiteten" eg here, here and here. Mr Picardt is considered the founding father of the study of archaeology in the Netherlands. The drawings seem to show the hunebedden being built by giants and dwarfs. But the dwarfs seem to get the raw end of the deal as the giants end up eating them. That's certainly what it looks like at any rate.
Throughout Europe and even adjacent areas there was the widespread belief in thunderstones. These peculiar stones (prehistoric flint and stone axes) were thought to have crashed into the earth during a lightning strike. Although nowadays this superstition has largely vanished, it was still widely accepted in the first half of the 20th century.
Deinse* describes this situation for the Dutch province of Overijssel, directly south of Drenthe. He reports that virtually every farmer has at least one prehistoric axe at his farm. They were believed to protect the house against lightning, as lightning never strikes the same place twice. He even reported that particular axes were believed to possess special powers. Small bits of stone were scraped off these axes and were given to children as a medicine against convulsions.
Deinse, J.J. (1925): Uit het Land van Katoen en Heide - Oudheidkundige en Folkloristische schetsen uit Twente. p102-111
This is from p25 of 'Ceci n'est pas une hache. Neolithic Depositions in the Northern Netherlands' by Karsten Wentink, 2006 - which you can read online at Google Books - it has lots of Serious archaeological information and discussion in it.
Acknowledged as the smallest of the passage graves in The Netherlands, hunebed D22 Bronneger lies barely 20 metres east of its twin, D21, overshadowed by an ancient oak tree that has seen better days. D22 originally consisted of just two large capstones supported on five supporting stones, but only the former can be seen.
When the hunebed was excavated in 1918 by Albert van Giffen, he discovered the three sidestones and two endstones still in place but almost entirely hidden below their covering of sand. The excavation also revealed 41 items of pottery.
Hunebed D21 Bronneger, which lies below the canopy of an impressive, mature beech tree, is the most beautiful of this group of five passage graves, all situated about a kilometre west of the village of Bronneger. It is also the most important on account of the archaeological finds made there by Albert van Giffen during his thorough 1918 excavations.
At that time, the hunebed was missing one of its sidestones, but as the work progressed, it was discovered lying flat on the floor of the grave, a metre below ground level, and re-erected in its rightful position. In total, D21 possesses three very large capstones resting on eight sidestones and two endstones, only the tips of which peek above the surrounding sand.
Discoveries made at the site included some complete Funnelbeaker pots as well as shards of around 600 other items of pottery. The floor of the grave consisted of several layers, between 1.5 and 1.7 metres below the capstones, a height clearly designed to allow those entering the chamber to stand upright inside it, and it was between these layers that the artefacts were discovered.
One of the smaller hunebedden, D15 Loon, with its ten sidestones and five capstones (four intact but the middle one reduced to a broken fragment lying in the crypt), ranks as one of the most complete passage graves in the Netherlands. It also retains 18 of an original total of 23 kerbstones, and is in fact the shortest hunebed known to have been surrounded by a kerb. These days, only 14 of the hunebedden retain one or more kerbstones. This hunebed also possesses a well defined entrance passage with two pairs of sidestones and a capstone.
Up until March 1870, D15 Loon was still covered by its barrow, which reached right up to the base of the capstones. Unfortunately, it was at that time that the barrow was removed in a mistaken attempt at 'restoration': this was carried out in the belief that the mound consisted of sand that had built up around the hunebed over the centuries, and was not part of the original monument. This event also resulted in the destruction of much of the earthenware that had been buried in the crypt.
In 1974, a beautiful earthenware jar, a beaker and a piece of bronze—all dating from the Funnelbeaker period (2450-2000 BCE)—were illegally unearthed from the entranceway by two youths from Assen, but were fortunately recovered and placed in the Drenthe Museum.
The bus from Assen to Groningen (Bus No 58) takes you to Loon in just 10 minutes. From the bus stop in the centre of the village, follow the main road (Gasterenseweg) for about 400 metres, then take the first turn off on the left (Heirweg) and continue for another 400 metres. You will see Hunebed D15 Loon ahead on the right long before you reach it.
Hunebed D16 Balloo is fairly complete, with nine capstones resting on 19 sidestones. There is also a complete entrance portal comprising two sidestones and a capstone, as well as a few of the original kerbstones. When Albert van Giffen made his inventory of D16 in 1918, the dolmen was in very poor condition, all nine of the capstones having slid down into the grave.
In the 1978 restoration, these were all put back in place but, over the years that followed, first one, then a second fell off again. At the beginning of December 1999, both these capstones were reinstated and secured with steel pins. In late June 2000, another capstone was dislodged from its supporting sidestones, apparently, deliberately by force, and all three of these stones now have large splinters broken from them. The damage was made good later that year.
An interesting feature of D16 Balloo is that capstone No 6 has a set of six cupmarks on its upper surface. D16 is the only Dutch hunebed on which such cupmarks are found.
You can reach the road that takes you to D16 in 10 minutes by taking Bus 24 from Assen station to Rolde, and alighting a little before the village of Rolde—at bus-stop Weg naar Balloo. From here, follow Tumuliboslaan northwards through an avenue of trees for 200 metres and branch left on to a sandy track called Weg voor de Strengen. After 150 metres take the path to the right through Kampsheide, staying on the same track for just over half a kilometre, where you will find the hunebed among scattered trees at the edge of the field to your right (Total walking distance is 1.5 kilometres. Takes 25 minutes).
You can view a short YouTube video about D16 Balloo, which includes a sequence showing the cupmarks.